1. Field of the Invention
The present invention relates to a flat cable for electrically connecting a fixed body and a rotating body which can be freely rotated coaxially to this fixed body.
2. Description of the Prior Art
For example, in a steering installation in an automobile or the like, a flat cable comprising plural conductor strips sandwiched between polyethylene terephthalate sheets or the like is spirally put into a space made between a rotating body fitted to a steering wheel and a fixed body fitted to a bearing of a steering shaft. An electrically connecting device having a structure wherein the rotating body and the fixed body are electrically connected (for example, in an air bag circuit) by means of the above-mentioned flat cable is fitted up to the steering installation.
Incidentally, in the electrical connecting device, the space around the steering wheel for accommodating the flat cable is limited. The width of the flat cable is also limited. For this reason, if the number of necessary circuits becomes larger than the number of the conductor strips in the flat cable, it is necessary to increase the number of the conductor strips by some other methods.
Thus, as the means for solving the above-mentioned problem, there is suggested, for example, a means disclosed in Japanese Utility Model Application. Laid Open No. 63-70684. As shown in FIG. 1, in this means the number of conductor strips is increased by winding plural flat cables 103 into layers. In each of the cables 103, plural conductor strips 101, made of copper foil or the like, are put between base films 102 made of polyethylene terephthalate (PET) or the like, so as to be integrated.
However, when the flat cables 103 are wound into layers, the flat cables 103 move freely inside the above-mentioned space. Therefore, base films 102 on the flat cables 103 may be worn away by friction between the cables 103. If the degree of the wear is considerable, a risk that the conductor strips 101 are bared so as to be short-circuited is incurred.
As shown in FIGS. 2 and 3, Japanese Utility Model Application, Laid Open No. 1-148687 also suggests a flat cable 103 in which plural conductor strips 101 are disposed onto the front and back surfaces of an inside base film 102 and then this inside base film 102 is sandwiched between outside base films 102 so as to be bonded and integrated thereto from upper and lower directions, thereby increasing the number of the conductor strips.
However, in the flat cable 103 having the above-mentioned structure, the conductor strips 101, made of rolled copper foil (having poor flexibility) or the like, are sandwiched between the base films 102, using an adhesive agent. The flat cable 103 is wound in a roll-to-roll manner, and cut. In this way, the flat cable 103 is produced. Therefore, circuits made by the conductor strips 101 become linear. The shape of the circuits is much limited. Moreover, the cable 103 has only poor flexibility.
Therefore, as shown in, for example, FIG. 4A, in order to connect the flat cable 103 to an external harness, it is necessary to use buss bars 104 having a wider pitch than the arranged pitch of the conductor strips 101 so as to connect the cable 103 to a partner connector. Alternatively, as shown in FIG. 4B, it is necessary to use xe2x80x9cLxe2x80x9d-shaped buss bars 104xe2x80x2.
As described above, the prior art has difficulties in realization of a multi-circuit and in production of a circuit having a complicated shape.
Thus, an object of the present invention is to provide a highly reliable flat cable making it possible to keep electric insulation, realize a multi-circuit, produce a circuit having a complicated shape and make its thickness thin.
To achieve the object, according to a first aspect of the present invention, there is provided a flat cable for electrically connecting a fixed body and a rotating body which can be freely rotated coaxially to this fixed body, comprising: a first insulating film on which plural conductor strips which are formed by plating and etching processes are disposed; and a second insulating film on which plural conductor strips which are formed by plating and etching processes are disposed; wherein the conductor strips are sandwiched between the first and second insulating films so as to be made multilayer; and the conductor strips disposed on the first insulating film and the conductor strips disposed on the second insulating film are layered so as not to be electrically connected to each other.
In the flat cable of the first aspect, the conductor strips are formed by the plating and etching processes, and the conductor strips are put together in two or more layers. The layered conductor strips are sandwiched between the insulating films so as to be made multilayered. Therefore, as compared with flat cables having a structure wherein rolled copper foil is merely sandwiched between base films, the shape of the conductor strips can be more freely varied and can be made more complicated in the present invention. Moreover, the flexibility thereof is as good as flexible printed-wiring boards.
According to a second aspect of the present invention, there is provided a flat cable for electrically connecting a fixed body and a rotating body which can be freely rotated coaxially to this fixed body, comprising: an insulating film being provided with plural conductor strips formed by plating and etching processes on each of front and back surfaces thereof; and a solder resist being screen-printed so as to cover the conductor strips.
In the flat cable of the second aspect, the solder resist is screen-printed to cover the conductor strips. As a result, the thickness of the cable itself can be made thin. Moreover, the conductor strips can be freely changed into various shapes and can be made into complicated shapes since the conductor strips are formed by the plating and etching processes.
According to a third aspect of the present invention, as it depends from the first or the second aspect, wherein at least one end portion of each of the conductor strips is bent.
In the flat cable of the third aspect, by bending the end portions of the conductor strips, the flat cable can be directly connected to a partner connector or the like without use of buss bars made into an xe2x80x9cLxe2x80x9d-shaped form or the like form.
According to a fourth aspect of the present invention, as it depends from one aspect among the first aspect to the third aspect, wherein the interval between the arranged conductor strips in the bent end portion is wider than the interval between the arranged conductor strips in the other portions in the flat cable.
In the flat cable of the fourth aspect, the conductor strips can be freely made into any shape. Therefore, by making the interval between the arranged conductor strips in the bent end portions wider than that in the other portions, no buss bars become necessary. It is also possible to make the width of a flexible mobile portion accommodated in a restricted space narrow and make the width of an external terminal connecting portion wide.